1. Field of the Invention:
This invention relates to an anti-skid control apparatus for a vehicle braking system which can prevent locking of the wheels.
2. Description of the Prior Art:
An anti-skid control apparatus for a vehicle braking system is known that includes fluid pressure control valve devices arranged between fluid pressure generating chambers of a tandem master cylinder and the wheel cylinders of the front wheels, respectively, and a control unit receiving outputs of wheel speed sensors to measure skid conditions of the front and rear wheels and to generate instructions for controlling the fluid pressure control valve devices.
When a fluid pressure control valve device is provided for each of four wheels (four channels), and their fluid pressures are independently controlled, there is no problem of controlling operation. Or when a fluid pressure control valve device is provided for each of the front wheels and for both of the rear wheels in common (three channels), there is no problem of controlling operation. In the latter case, the one common fluid pressure control valve device is controlled on the basis of the lower one of the speeds of the rear wheels.
However, the above case require three or four fluid pressure control valve devices. Accordingly, the whole anti-skid control apparatus is big and very heavy. Since the fluid pressure control valve device is expensive, the cost is high.
This Applicant previously proposed (Japanese Patent Application No. 247146/1986) in consideration of the above problems and in order to provide an anti-skid control apparatus for a vehicle braking system which can be of the two-channel type, small-sized and light, and can be stable in steering. This anti-skid control apparatus for a vehicle braking system includes (A) a pair of front wheels and a pair of rear wheels; (B) wheel speed sensors associated with the wheels, respectively; (C) a first fluid pressure control valve device for controlling the brake fluid pressure of the wheel cylinder of one of the front wheels, arranged between a first fluid pressure generating chamber of a tandem master cylinder and the wheel cylinder of the one front wheel; (D) a second fluid pressure control valve device for controlling the brake fluid pressure of the wheel cylinder of another of the front wheels, arranged between a second fluid pressure generating chamber of said tandem master cylinder and said wheel cylinder of the other front wheel; and (E) a control unit receiving outputs of the wheel speed sensors to measure the skid conditions of the front and rear wheels and to generate instructions for controlling the first and second fluid pressure control valve devices. The control unit discriminates the frictionally lower one (designated as "low side") of the sides of the road on which the wheels are running on the basis of measuring the skid conditions of the rear and/or front wheels and combines logically the measurement of the skid condition of the one rear wheel running on said low side of the road with that of the one front wheel running on the low side and generates an instruction for controlling the first or second fluid pressure control valve device for the corresponding front wheel. It also generates the instruction for controlling the second or first fluid pressure control valve device for the other front wheel, on the basis of the measurement of the skid condition of the other front wheel running on the high side (frictionally higher side) independently of those of the rear wheels. This anti-skid control apparatus for a vehicle braking system includes (A) a pair of front wheels, and a pair of rear wheels; (B) wheel speed sensors associated with the wheels, respectively; (C) a first fluid pressure control valve device for controlling the brake fluid pressure of the wheel cylinder of one of the front wheels, arranged between a first fluid pressure generating chamber of a tandem master cylinder and the wheel cylinder of the one front wheel; (D) a second fluid pressure control valve device for controlling the brake fluid pressure of the wheel cylinder of another of the front wheels, arranged between a second fluid pressure generating chamber of the tandem master cylinder and said wheel cylinder of the other front wheel; and (E) a control unit receiving outputs of said wheel speed sensors for measuring or judging the skid conditions of said front and rear wheels and for generating instructions for controlling said first and second fluid pressure control valve devices. In this devices the control unit selects the frictionally lower (designated as "low side") of the sides of the road on which the wheels are running on the basis of measuring the skid conditions of the rear and/or front wheels and combining logically the measurement of the skid conditions of the rear wheels with the measurement of the skid condition of the one front wheel running on the low side for generating the instruction for controlling said first or second fluid pressure control valve device for the corresponding front wheel, and generates the instruction for controlling said second or first fluid pressure control valve device for the other front wheel, on the basis of the measuring or judging result of the skid condition of the other front wheel running on the high side (frictionally higher side) independently of those of the rear wheels.
In the above-described apparatus, the brake relieving instruction for decreasing the brake fluid pressure, on the basis of the instructions for controlling the first or second fluid pressure control valve device, obtained from the logical combination of the judging results, is formed by the logical sum (OR) of the brake relieving instructions for the one front wheel and rear wheel on the same low side or for the one front wheel on the low side and both rear wheels.
In a 4WD vehicle, the rotational speeds of all wheels are coupled together mechanically and are substantially identical when the vehicle is moving in a straight line and is not being braked. When the anti-skid control is in operation, wheels that are driven by the same differential transmit torque through the differential to each other with a time delay of, for example, about 100 milliseconds. The same effect is apparent when two drive shafts are driven by the same differential. Thus, when one wheel is slowed more than the other that is on its differential, the other wheel receives more torque, increasing its rotational speed, but with the above time delay. This time delay is added to the brake relieving time, which reduces braking effectiveness.